The number of patients with implanted pacemakers, cardioverters, defibrillators, neural stimulators, and the like is steadily increasing around the world, and is likely to continue to do so. A great many of these patients are partially or completely dependent upon their implanted devices, and it is very important for implanted devices to operate reliably. It has always been desirable for a clinician to be able to readily obtain reliable information about various aspects of an implanted device's operational status. The status of the device's power supply is of particular concern.
The most prevalent source of power used in implanted devices is battery power, very often from lithium-iodine cells. Many systems have been proposed in the prior art for providing some sort of battery end-of-life (EOL) indication when the device's power supply is nearly depleted. Of course, it is desirable for the circuitry for generating the EOL indication to be able to anticipate battery depletion early enough to allow time for appropriate remedial action to be taken, such as replacement of the device.
With the advent of improved batteries which can last for many years before depletion, the need has arisen for more information than a simple EOL indication. For example, it has been deemed desirable for the device to provide information regarding how much longer the battery will last under the normal demand that is placed on it.
Lithium-iodine batteries have a characteristic feature that their internal resistance curve is substantially linear as a function of energy depletion until near EOL, at which time the curve exhibits a "knee" where internal resistance begins to rise rapidly. In lithium-iodine batteries, the cell cathode consists of molecular iodine weakly bonded to polyvinyl pyridine (P2VP). The initial cathode composition of lithium-iodine batteries is often expressed as the weight ratio of I.sub.2 to P2 VP. Typical values of this ratio range from 20:1 to 50:1 . No electrolyte as such is included in the construction of the cell, but a lithium iodine (LiI) electrolyte layer forms during cell discharge, between the anode and the cathode. The LiI layer presents an effective internal resistance to Li+ ions which travel through it. Since the LiI layer grows with the charge drawn from the battery, this component of the battery resistance increases linearly as a function of energy depletion. In the implantable device context, where there is typically a relatively continuous energy depletion, this component of the internal resistance increases continually over time. However, particularly for a demand type pacemaker which at any given time may or may not be called upon to deliver stimulating pulses, the increase in this component is continuous but not necessarily linear with time, due to the fact that current drain is not constant.
Another component of internal resistance in lithium-iodine cells is caused by depletion of iodine in the cathode. The cathode is essentially a charge transfer complex of iodine and P2VP, and during discharge of the cell iodine is extracted from this complex. As noted above, the weight ratio of I.sub.2 to P2VP at beginning of life may range from 20:1 to 50:1. During extraction of iodine from the complex, the resistance to this process is low until the point is reached where the I.sub.2 -to-P2VP ratio is reduced to approximately 8:1, the ratio at which the cathode becomes a single phase and the iodine activity begins to be less than unity. At this point the resistance rises sharply. This gives rise to a non-linear internal resistance component which, for the lithium-iodine cell, is called variously the depletion resistance, depolarizer resistance, the charge-transfer complex resistance, or the pyridine resistance. By whatever names, the combination of the non-linear component with the linear component produces an overall resistance curve with a knee occurring toward EOL, the knee being caused by the reaching of the depletion of available charge carriers from the cathode.
In the prior art, some EOL indicator arrangements in implantable devices evaluate battery life based simply upon the terminal voltage of the battery, indicating EOL when the voltage falls below a predetermined threshold. However, due to the internal impedance of the battery, terminal voltage varies significantly depending upon current consumption. Thus, if relatively little current is drawn from the battery for a period of time when the battery is nearing but has not reached its EOL, a sudden prolonged period of high demand on the battery may cause a situation in which too little time is available between indication of EOL and total depletion of the battery. For a particular pacemaker and electrode combination in a given patient, there will be a variation in the effective load on the lithium-iodine battery, and a resulting variation in the overall current drain. Accordingly, if an EOL indication is predicated upon sensing the voltage of the battery and detecting when it drops below a certain level, there can be very little assurance that the level chosen will correspond to the knee of the internal resistance curve.
It has been recognized in the prior art that since remaining battery life is directly related to the internal impedance of the battery itself, remaining battery life can be reliably predicted through accurate measurement of internal battery impedance. In U.S. Pat. No. 5,137,020 issued to Wayne et al. and assigned to the assignee of the present invention, there is described a battery impedance measuring arrangement wherein a current source and a reference impedance are applied to a battery which has been isolated from the remainder of the pacemaker circuitry. The Wayne et al. '020patent is hereby incorporated by reference in its entirety into the present disclosure.
Other battery impedance measuring arrangements are proposed, for example, in U.S. Pat. Nos. 4,259,639 to Renirie, 4,231,027 to Mann et al., and 4,324,251 to Mann. These patents are also hereby incorporated by reference herein in their entirety. The theory underlying the use of internal impedance as a EOL warning indicator is that at low current drains typical of implantable medical devices, plots of resistance versus time give more warning than plots of terminal voltage over time. If voltage characteristics for different current drains are considered, the knees in the impedance curve are observed to have a fairly wide variation, meaning that the voltage at which the knee might appear is similarly subject to substantial variation as a function not only of the particular battery being used but also of the current being drawn by the pacemaker circuitry at a given time. On the other hand, plots of resistance indicate that the knee varies over a smaller range of values of internal resistance. Since the current drain may vary drastically with different electrode loads, the variation in voltage may be many times the variation in internal resistance. Monitoring the internal resistance thus provides a more direct indication of the depth of discharge of the battery, whereas monitoring the output voltage gives a much less direct indication, reflecting not only the depth of discharge but also the current drain.
Although the internal impedance of an implanted battery is believed to more accurately reflect the level of battery depletion than the battery voltage, errors still exist. For example, open circuit battery voltage typically changes by 28-mV or so from beginning-of-life (BOL) to end-of-life. This leads to a typical 5% error in an expected 20-k.OMEGA. end-of-life impedance level. It is one feature of the present invention, therefore, that such variation in open-circuit battery voltage is accounted for in the computation of a battery's internal resistance.